Lecture 12.0Lecture 12.0

Deposition

Materials DepositedMaterials Deposited

Dielectrics– SiO2, BSG

Metals– W, Cu, Al

Semiconductors– Poly silicon (doped)

Barrier Layers– Nitrides (TaN, TiN), Silicides (WSi2, TaSi2, CoSi,

MoSi2)

Deposition MethodsDeposition Methods

Growth of an oxidation layer Spin on Layer Chemical Vapor Deposition (CVD)

– Heat = decomposition T of gasses– Plasma enhanced CVD (lower T process)

Physical Deposition– Vapor Deposition– Sputtering

Critical IssuesCritical Issues

Adherence of the layerChemical Compatibility

– Electro Migration– Inter diffusion during subsequent

processing • Strong function of Processing

Even Deposition at all wafer locations

CVD of SiCVD of Si33NN44 - Implantation mask - Implantation mask

3 SiH2Cl2 + 4 NH3Si3N4 + 6 HCl + 6 H2

– 780C, vacuum

– Carrier gas with NH3 / SiH2Cl2 >>1

Stack of wafer into furnace– Higher temperature at exit to compensate for

gas conversion losses

Add gases Stop after layer is thick enough

CVD of Poly Si – Gate conductorCVD of Poly Si – Gate conductor

SiH4 Si + 2 H2

– 620C, vacuum

– N2 Carrier gas with SiH4 and dopant precursor

Stack of wafer into furnace– Higher temperature at exit to compensate for

gas conversion losses

Add gases Stop after layer is thick enough

CVD of SiOCVD of SiO22 – Dielectric – Dielectric

Si0C2H5 +O2SiO2 + 2 H2

– 400C, vacuum– He carrier gas with vaporized(or atomized)

Si0C2H5 and O2 and B(CH3)3 and/or P(CH3)3 dopants for BSG and BPSG

Stack of wafer into furnace– Higher temperature at exit to compensate for

gas conversion losses Add gases Stop after layer is thick enough

CVD of W – Metal plugsCVD of W – Metal plugs

3H2+WF6 W + 6HF– T>800C, vacuum– He carrier gas with WF6

– Side Reactions at lower temperatures• Oxide etching reactions• 2H2+2WF6+3SiO2 3SiF4 + 2WO2 + 2H2O• SiO2 + 4HF 2H2O +SiF4

Stack of wafer into furnace– Higher temperature at exit to compensate for gas conversion

losses Add gases Stop after layer is thick enough

Chemical EquilibriumChemical Equilibrium

CVD ReactorCVD Reactor

Wafers in Carriage (Quartz)

Gasses enterPumped out via

vacuum systemPlug Flow

Reactor

Vacuum

CVD ReactorCVD Reactor

Macroscopic Analysis– Plug flow reactor

Microscopic Analysis– Surface Reaction

• Film Growth Rate

Macroscopic AnalysisMacroscopic Analysis

Plug Flow Reactor (PFR)– Like a Catalytic PFR Reactor– FAo= Reactant Molar Flow

Rate– X = conversion– rA=Reaction rate = f(CA)=kCA

– Ci=Concentration of Species, i.– Θi= Initial molar ratio for species i

to reactant, A.– νi= stoichiometeric coefficient– ε = change in number of moles

TR

PC

T

T

P

P

X

XCC

V

AXr

dXFV

g

AoAo

o

o

iiioi

X

reactor

waferA

Aoreactor

1

)(0

'

Combined EffectsCombined Effects

Contours = Concentration

Reactor LengthReactor Length Effects Effects

SiH2Cl2(g) + 2 N2O(g) SiO2(s)+ 2 N2(g)+2 HCl(g)

nwafer VReactorPerWafer a

FAo0

X

X1

r'A X( )

d n X( )FAo

VReactorPerWafer a 0

X

X1

r'A X( )

d

rate X( )

r'A X( )4

Dwafer2

SiO2

MwSiO2Awafer

0 50 100 1500

2000

4000

6000

Wafer Number

Th

ick

ness

(nm

)

rate X'( ) 10 minnm

n X'( )0 0.5 10

200

400

600

Conversion

Dep

osi

tio

n R

ate

, W

afe

r N

um

ber

rate X( )

nm

min

n X( )

X

How to solve? Higher T at exit!

Deposition Rate over the RadiusDeposition Rate over the Radius

r

wAsA

A

pABe

wA

Ae

RrCC

rfiniteC

ConditionsBoundary

DD

V

Ar

dr

CdrD

dr

d

r

,

0,

1 "

CAs

Thiele Modulus Φ1=(2kRw/DABx)1/2

Radial EffectsRadial Effects

This is bad!!!

Pseudo First Order Results

CA 1

sinh 1 sinh 1

00.510.97

0.98

0.99

1

r/R.wafer

Con

cent

rati

on

CA

00.51

4900

4950

5000

5050

r/R.wafer

Thi

ckne

ss(n

m)

rate 1 CA 10 min

nm

x 0.5

Combined Length and Radial EffectsCombined Length and Radial Effects

00.512400

2600

2800

3000

3200

3400

3600

r/R.wafer

Th

ick

ness

Rate 10 10 minnm

Rate 20 10 minnm

Wafer 20

Wafer 10

CVD ReactorCVD Reactor

External Convective Diffusion– Either reactants or products

Internal Diffusion in Wafer Stack– Either reactants or products

AdsorptionSurface ReactionDesorption

Microscopic Analysis -Reaction StepsMicroscopic Analysis -Reaction Steps

Adsorption – A(g)+SA*S– rAD=kAD (PACv-CA*S/KAD)

Surface Reaction-1 – A*S+SS*S + C*S

– rS=kS(CvCA*S - Cv CC*S/KS) Surface Reaction-2

– A*S+B*SS*S+C*S+P(g)– rS=kS(CA*SCB*S - Cv CC*SPP/KS)

Desorption: C*S<----> C(g) +S– rD=kD(CC*S-PCCv/KD)

Any can be rate determining! Others in Equilib. Write in terms of gas pressures, total site conc.

Rate Limiting StepsRate Limiting Steps

Adsorption– rA=rAD= kADCt (PA- PC /Ke)/(1+KAPA+PC/KD+KIPI)

Surface Reaction – (see next slide)

Desorption– rA=rD=kDCt(PA - PC/Ke)/(1+KAPA+PC/KD+KIPI)

Surface ReactionsSurface Reactions

Deposition of GeDeposition of Ge

3"

22

22

1 HHGeClA

HGeClHAsDep

PKPK

PPKKkr

Ishii, H. and Takahashik Y., J. Electrochem. Soc. 135,1539(1988).

Silicon DepositionSilicon Deposition

Overall Reaction– SiH4 Si(s) + 2H2

Two Step Reaction Mechanism– SiH4 SiH2(ads) + H2

– SiH2 (ads) Si(s) + H2

Rate=kadsCt PSiH4/(1+Ks PSiH4)

– Kads Ct = 2.7 x 10-12 mol/(cm2 s Pa)

– Ks=0.73 Pa-1

Silicon Epitaxy vs. Poly SiSilicon Epitaxy vs. Poly Si

Substrate has Similar Crystal Structure and lattice spacing– Homo epitaxy Si on Si– Hetero epitaxy GaAs on Si

Must have latice match– Substrate cut as specific angle to assure latice match

Probability of adatoms getting together to form stable nuclei or islands is lower that the probability of adatoms migrating to a step for incorporation into crystal lattice.– Decrease temp.– Low PSiH4

– Miss Orientation angle

Surface DiffusionSurface Diffusion

Monocrystal vs. PolycrystallineMonocrystal vs. Polycrystalline

PSiH4=? torr

Dislocation DensityDislocation Density

Epitaxial Film– Activation

Energy of Dislocation

• 3.5 eV

Physical Vapor DepositionPhysical Vapor Deposition

Evaporation from Crystal

Deposition of Wall

Physical Deposition - SputteringPhysical Deposition - Sputtering

Plasma is usedIon (Ar+) accelerated into a target

materialTarget material is vaporized

– Target Flux Ion Flux* Sputtering YieldDiffuses from target to waferDeposits on cold surface of wafer

DC PlasmaDC Plasma

Glow Discharge

RF Plasma Sputtering for RF Plasma Sputtering for Deposition and for EtchingDeposition and for Etching

RF + DC field

Sputtering ChemistriesSputtering Chemistries

Target– Al– Cu– TiW– TiN

Gas– Argon

Deposited Layer– Al– Cu– TiW– TiN

Poly Crystalline Columnar Structure

Deposition RateDeposition Rate

Sputtering Yield, S– S=α(E1/2-Eth

1/2)

Deposition Rate – Ion current into Target *Sputtering Yield– Fundamental Charge

gas(x) andtarget(t) ofnumbersatomic

)(

2.53/2

4/33/23/2

i

xt

x

xt

t

Z

energybindingsurfaceU

ZZ

Z

ZZ

Z

U

RF PlasmaRF Plasma

Electrons dominate in the Plasma– Plasma Potential, Vp=0.5(Va+Vdc)– Va = applied voltage amplitude (rf)

Ions Dominate in the Sheath– Sheath Potential, Vsp=Vp-Vdc

Reference Voltage is ground such that Vdc is negative

Plasma rfSheath

Sheath

Floating PotentialFloating Potential

Sheath surrounds objectFloating potential, Vf

kBTe=eV – due to the accelerating Voltage

eTemperaturelectronT

3.2ln

2q

Tk -VV

e

eBpf

e

i

m

M

Plasma ChemistryPlasma Chemistry

Dissociation leading to reactive neutrals

– e + H2 H + H + e

– e + SiH4 SiH2 + H2 + e

– e + CF4 CF3 + F + e

– Reaction rate depends upon electron density

– Most Probable reaction depends on lowest dissociation energy.

Plasma Chemistry Plasma Chemistry

Ionization leading to ion– e + CF4 CF3

- + F

– e + SiH4 SiH3+ + H + 2e

Reaction depend upon electron density

Plasma ChemistryPlasma Chemistry

Electrons have more energyConcentration of electrons is ~108 to

1012 1/ccIons and neutrals have 1/100 lower

energy than electronsConcentration of neutrals is 1000x

the concentration of ions

Oxygen PlasmaOxygen Plasma

Reactive Species– O2+eO2

+ + 2e

– O2+e2O + e

– O + e O-

– O2+ + e 2O

Plasma ChemistryPlasma Chemistry

Reactions occur at the Chip Surface– Catalytic Reaction Mechanisms

– Adsorption– Surface Reaction– Desorption

• e.g. Langmuir-Hinshelwood Mechanism

Plasma Transport EquationsPlasma Transport Equations

Flux, J

mobilityelectronμ

mobilityionμ

e

i

electronsforEndx

dnDJ

ionsforEndx

dnDJ

neutralsfordx

dnDJ

eee

ee

iii

ii

nnn